The present invention relates generally to screw compressors, and, more specifically, to rotary actuators therefor.
A screw compressor has advantages in compressing gases in various applications such as gas production and distribution, landfill gas, fuel gas boosters, and refrigeration systems. All of these applications are subject to various degrees of hostile environment in which the compressors must operate with precision and reliability.
A typical single screw compressor includes a cylindrical main screw with helical grooves therein which cooperate with a pair of flat star shaped gate rotors that mesh therewith. The helically fluted screw is driven by a motor during operation, and gas is compressed along each helical groove as a corresponding tooth of the gate rotor travels therealong.
The two gate rotors are disposed on opposite sides of the main screw for compressing the gas simultaneously in two flowpaths. Capacity and volume ratio may be separately controlled in each flowpath of the screw pump using corresponding slide valves. The slide valves in turn are driven by corresponding rack-and-pinion gearboxes which convert rotary motion at the pinion to axial translation of the rack which drives the corresponding slide valves.
The pinion, in turn, for each control valve is driven by a rotary actuator. The rotary actuator includes an electrical motor driving a reduction gearbox having an outlet shaft coupled to the pinion. An electrical controller for the screw compressor controls operation of the individual rotary actuators for positioning the corresponding slide valves between minimum and maximum slide positions as operation requires.
Precise control of the slide valves requires precise control of the rotary actuators. The actuator motors typically operate at constant speed, with corresponding minimum and maximum rotary positions of the output shafts. Shaft feedback position is normally obtained using a conventional rotary potentiometer which varies in electrical resistance as its rotor is turned. The measured resistance of the potentiometer may be calibrated against the rotary position of the actuator output shaft in a relatively complex procedure.
More specifically, normal rotary potentiometers have a maximum rotary range of about 270 degrees, whereas the required rotary range of the actuator output shaft is typically greater than 270 degrees. Accordingly, a reduction gearset must be used to operatively join the potentiometer to the actuator output shaft, and correspondingly calibrated to ensure that the rotary range of the potentiometer matches the rotary range of the output shaft. Such actuators with rotary potentiometers and cooperating gear sets have been in public use in the U.S. for many years, and commercially available from several sources, such as El-O-Matic International, Hackensack, N.J., for example.
Since the typical environment of the screw compressor is hostile and is subject to heat, vibration, and oil, potentiometers are unreliable and experience a high failure rate.
Accordingly, it is desired to provide an improved rotary actuator for use with screw compressors, as well as for other applications requiring precise control of rotary position.
A rotary actuator includes a motor operatively joined to a gear train having an output shaft. An optical encoder is operatively joined to the shaft of the motor for detecting rotation thereof. A controller is operatively joined to the encoder, and is configured for operating the motor to drive the output shaft between minimum and maximum rotary positions as detected by the encoder.